Sarcomere in striate muscle is the basic unit of contractile apparatuses. Assembly and maintenance of organized sarcomeric structure are essential for producing contractile forces. Actin is one of the major components of sarcomeric thin filaments, and length and orientation of the filaments are precisely regulated in striated muscle. However, the mechanism of assembly and maintenance of sarcomeric actin filaments is complex and poorly understood. What is more puzzling is that actin subunits within thin filaments are dynamically exchanged without compromising overall structure and contractile functions. A number of regulators of actin dynamics have been identified in skeletal muscle, and some of them are linked to genetic muscle disorders. In particular, nemaline myopathy involves formation of abnormal actin-rich aggregates or rods in skeletal muscle and is caused by mutations in actin or regulators of actin dynamics including nebulin, tropomyosin, and cofilin. Therefore, the regulation of actin dynamics is fundamentally important for building functional contractile apparatuses in skeletal muscle, and malfunction in this system leads to muscle disorders. To investigate the regulatory mechanism of actin dynamics in striated muscle, we have been using the nematode Caenorhabditis elegans as a model system. Body wall muscle of C. elegans is striated muscle, and most of sarcomeric proteins are conserved between C. elegans and humans. Powerful genetic studies on these muscle proteins advanced our knowledge on the mechanism of myofibril assembly and function. Using this system, we have identified that ADF/cofilin and AIP1 enhance turnover of actin filaments and essential for organized assembly of sarcomeric actin filaments, and that tropomyosin and calponin-like protein antagonize ADF/cofilin and stabilize sarcomeric actin filaments. These findings led us to hypothesize that a balance between enhancers and suppressors of actin dynamics is crucial for sarcomere assembly and maintenance. Recently, we obtained evidence that additional regulators of actin dynamics are involved in sarcomeric actin regulation. Therefore, we propose to further investigate their roles in myofibril assembly in three specific aims: (Aim 1) We identified a cyclase-associated protein isoform that is enriched in body wall muscle and will determine how cyclase-associated protein cooperate with ADF/cofilin and AIP1 to regulate actin turnover, (Aim 2) We identified a gelsolin-like protein with strong actin-severing activity that is enriched in body wall muscle and will investigate its role in actin organization, and (Aim 3) We demonstrated distinct localization patterns of actin-regulatory proteins near the pointed ends of sarcomeric thin filaments and will investigate how actin dynamics are regulated in this specialized region. We expect that results of this research will provide new insight into the regulation of actin dynamics in striated muscle.